The present invention relates to a magnetic recording medium.
Magnetic recording media prepared by coating a composition of a magnetic power and a binder on a substrate such as a plastic film e.g. a polyester film or an aluminum desk have been used in the past. Various binders have been proposed for these megnetic recording media. However, a completely suitable binder for use in the preparation of media has not been found. A nitrocellulose composition has been used as the binder because of its excellent wetting and excellent levelling characteristics in the preparation of a magnetic composition. However, nitrocellulose is flammable and precautions against fire must be taken during handling so that as a regulation, from 30 to 35 wt. % of isopropyl alcohol is blended with nitrocellulose to prevent explosions caused by fine particles dusts of nitrocellulose in the air. By this technique the safe handling and storage of nitrocellulose can be achieved.
When nitrocellulose is used as a binder for magnetic tapes the wear resistance and electromagnetic characteristics of the tape can be improved because of improved affinity between the binder and the magnetic powder and improved mechanical strength of the binder when a polyurethane is combined with nitrocellulose. In this instance a cross-linked structure is formed by reaction of hydroxyl groups in the nitrocellulose with isocyanate groups in the polyurethane. However, the isopropyl alcohol which is always present with the nitrocellulose interfers with the cross-linking reaction because isopropyl alcohol itself reacts with the isocyanate groups. Because the isopropyl alcohol interfers with the cross-linking reaction, the improved degree of wear resistance of the recording medium and running friction characteristics sought cannot be attained. These problems are discussed in U.S. Pat. No. 3,808,136 and U.S. Pat. No. 3,810,840.
In order to overcome these disadvantages, it has been proposed to improve the physical characteristics of the binder composition by blending and kneading the nitrocellulose composition with about 15 wt. % of a plasticizer such as dibutyl phthalate or dioctyl phthalate based on the solid component in a kneader and then vaporizing the isopropyl alcohol which is present in the nitrocelluous on a hot two roll-mill to prepare nitrocellulose composition chips in which the isopropyl alcohol is replaced by the plasticizer. The chips are then mixed with a polyurethane and mixing a crosslinking agent having active isocyanate groups so that the isocyanate groups can effectively react with the main binder components. However, the video tapes prepared by using such a binder have the disadvantages that troubles are encountered during running of the product tape or creaking sounds are heard which are caused by increased friction during running of the tape when it is used for a VTR. Moreover, the magnetic coating peels from the head in severe test for friction of the magnetic coating such as in steal reproduction tests for reproducing the same track to obtain a stationary image when the tape is stopped. This adverse effect results from the presence of the plasticizer such as dibutyl phthalate in the magnetic coating.